Speech processing systems such as automatic speech recognizers do not operate well in noisy environments. Consequently, microphone pickups for such systems must discriminate against noise and reverberation to maintain proper speech processing operation. Commonly used fixed microphone arrays with fixed directional response patterns may be preset to provide maximum response in one or more directions but such fixed arrays are highly susceptible to environmental interference due to sidelobe response and noise sources that change location. Some prior art speech recognition arrangements have avoided pickup of unwanted sounds through use of a special noise canceling microphone device close to the talker whose speech is to be recognized. While microphone placement very close to the speech source reduces the effect of background noise, the talker is required to hold the pickup device or wear a device containing the pickup device to maintain the close spacing. The talker's movements, however, may be restricted by the pickup device and he may be subjected to some discomfort. It is therefore desirable to provide a microphone arrangement that automatically adapts its directional response pattern to minimize unwanted sound pickup without being attached to or in close proximity to the talker.
U.S. Pat. No. 4,559,642 discloses phased array sound pickup apparatus that includes an array of directional microphones having individual directivity patterns equally oriented in a given direction. The directivity patterns of the microphones combine into the main lobe of the array. A plurality of variable delay circuits is connected to the microphones to provide incremental delays to the individual microphone signals and the delayed microphone signals are combined for delivery as an output of the apparatus. A delay control circuit is coupled to variable delay circuits to control the amount of the individual signal delays, and the delay control signals cause the main lobe to be steered to the given direction. In this manner, the need for close proximity between a talker and a sound pickup device is obviated. Adjustment of the delay signals, however, is performed manually and requires the services of a skilled operator. While the arrangement may be practical for large gatherings, it is uneconomical in speech processing applications such as those associated with computer terminals or personal computers where relatively few individuals are involved.
U.S. Pat. No. 4,536,887 describes microphone array apparatus and a method for extracting desired signals therefrom in which an acoustic signal is received by a plurality of microphone elements. The element outputs are delayed by delay means and weighted and summed up by weighted summation means to obtain a noise-reduced output. A "fictitious" desired signal is electrically generated and the weighting values of the weighted summation means are determined based on the fictitious desired signal and the outputs of the microphone elements when receiving only noise but no input signal. In this way, the adjustments are made without operator intervention. The requirement of an environment having substantially only noise sources, however, does not realistically reflect actual sound pickup situations where noise, reverberation and sound conditions change over relatively short time periods and the occurrence of desired sounds is unpredictable. It is an object of the invention to provide improved directional sound pickup that is adaptable to varying environmental conditions without operator intervention or a requirement of signal-free conditions for adaptation.
As disclosed in the article, "An Algorithm for Linearly Constrained Adaptive Array Processing", by O. L. Frost, III, Proceedings of the IEEE, Vol. 60, No. 8, August, 1972, pp. 926-935, further discussed below, in connection with FIG. 1, it is also known, in the field of fully automatic microphone array processing, how to reduce the effect of noise sources, while maintaining a chosen frequency characteristic for the array in the direction from which the signal is to be received, without the need for a signal in the microphone array circuitry to represent fictitiously the expected signal when none is present. Nevertheless, the constraints on the weighting of the signals from the various microphones of the array are so rigid, that frequently the Frost system will achieve very little reduction in noise. Therefore, it is an object of the invention to select such constraints to yield a better compromise of faithful reproduction of the signal and reduced noise power, while still not requiring the signal-free conditions and "fictitious desired signal" in the microphone array circuitry, as disclosed in U.S. Pat. No. 4,536,887.